1. Field of the Invention
The present invention relates, in general, to a method of fabricating a gallium nitride (GaN) substrate by which a GaN thick film is obtained without bending and cracks which may occur in the growing process.
2. Description of the Related Art
Generally, GaN has band gap energy of 3.39 eV and is a direct transition type semiconductor material, which is useful for fabricating a short-wavelength light-emitting device or the like. Since GaN single crystal has high nitrogen vapor pressure at a fusion point, it needs a processing condition of high temperature above 1500° C. and a nitrogen atmosphere of 20,000 atm to carry out liquid crystal growth. Thus, it is difficult to accomplish mass production thereof.
Until now, GaN film has been obtained on a hetero-substrate by using a vapor phase growing method, such as metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), or the like. In case of MOCVD, it can provide a high quality film but a growth rate thereof is too low, so that it is difficult to obtain a GaN substrate with a thickness of tens or hundreds micrometers. For the above reason, an HVPE growing method is mainly used to fabricate a GaN thick film.
As a hetero-substrate for fabricating a GaN film, a sapphire substrate is most widely used. This is because sapphire has the same hexagonal system as GaN, is cheap, and is stable at high temperature. However, strain is caused to a boundary due to differences of the lattice constant (about 13%) and thermal expansion coefficient (about 35%) between sapphire and GaN, generating defects and cracks in a crystal. This makes it difficult to grow a high quality GaN film and shortens the lifetime of a device fabricated on the GaN film.
When GaN is grown on a sapphire substrate, as illustrated in FIG. 1, bending occurs in the direction from the sapphire substrate 100 to a GaN layer 210 due to a difference in the thermal expansion coefficient between the sapphire substrate and the GaN layer. Meanwhile, in the cooling process after the growth of the GaN layer, as illustrated in FIG. 2, bending occurs in the counter direction and stress is applied all over the GaN layer. Even after the GaN layer is separated from the sapphire substrate, the durability of a GaN freestanding layer remains weak.
In order to prevent such bending, there has been proposed a method in which a GaAs substrate is used. GaAs has a thermal expansion coefficient less different from that of GaN than sapphire. However, GaAs is expensive and is weak with heat.
Alternatively, there has been proposed another method for preventing bending, in which a mask is formed (by sputtering, performing P-CVD, mask patterning, etching, etc.) between a base substrate and a GaN layer, or an oxide embedding layer is embedded therebetween. However, in these cases, since separate processes are needed, a fabricating process thereof becomes complicated, and thereby the fabricating cost and time increase.
Therefore, in order to fabricate a large-area GaN substrate at a high yield rate, it needs technology of reducing stress which is transferred from the base substrate to the GaN layer.